This application pertains to the art of video signal processing and, more particularly, to video speed logarithmic analog-to-digital converters. The invention is particularly applicable for use with electronic equipment using a television-type display or other video speed apparatus and will be described with particular reference thereto. It will, however, be appreciated that the invention has broader applications in other areas of high speed data acquisition and processing.
Conventionally, conversion of analog signals to logarithmic digital representations is performed as a two-step process. Most normally, analog signals are first converted with an analog logarithmic amplifier to an analog logarithmic representation. Then, the analog logarithmic representation is converted with a linear analog-to-digital converter to a digital representation of the analog logarithmic signal. The final results after a two-step process is a digital representation of the logarithm of the analog signal.
One of the principal problems with the prior logarithmic analog-to-digital converters is the relatively low speeds for making the conversion. The conversion time for converting an analog signal to a logarithmic digital representation with the prior art converters is typically several microseconds.
Another problem with prior logarithmic analog-to-digital converters is poor conformity with the logarithmic scale. Prior art logarithmic analog-to-digital converters frequently use linear approximations which deviate to some extent from the true logarithmic scale.
Yet another problem with prior logarithmic analog-to-digital converters has been a poor dynamic response. Logarithmic amplifiers have long decay transients relative to video speeds. These procedure relatively inaccurate analog signals for the linear analog-to-digital converter to convert.
The present invention contemplates a new and improved apparatus which overcomes all of the above-referred problems and others, yet provides high speed signal processing with moderately simple and inexpensive circuitry.
In accordance with the present invention, there is provided a high speed logarithmic analog-to-digital converter. It includes a multiple path logarithmic analog-to-digital converter in which each successive path converts an analog input signal to one or a group of progressively less significant bits of the logarithm. A novel series sample-and-hold circuits are also provided. The high speed sample-and-hold circuits are suited for use with high speed signal processing circuitry including, but not limited to, high speed logarithmic analog-to-digital converters as disclosed herein.
A principal advantage of the present invention is video speed conversion of analog data into logarithmic digital data.
A further advantage of the present invention is extremely accurate logarithmic conformance.
Other advantages include an extensive dynamic range and a digital output which is independent of prior analog inputs.